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Abstract

There is still significant speculation regarding the nature of femtosecond laser induced index change in bulk glasses with colour centre formation and densification the main candidates. In the work presented here, we fabricated waveguide Bragg gratings in doped and undoped phosphate glasses and use these as a diagnostic for monitoring subtle changes in the induced refractive index during photo- and thermal annealing experiments. Reductions in grating strengths during such experiments were attributed to the annihilation of colour centres.

Figures (6)

Fig. 1. (a) Experimental setup for annealing waveguide devices. WDM-Wavelength division multiplexor, T-Transmission, R-Reflection. Components inside the dotted blue box are removed from the setup for 976 nm ‘pumping’. (b) Sketch of the relative refractive index of the bulk, a waveguide and the grating contrast of a WBG. (c) Example of transmission data through a WBG before and after photo-annealing.

Fig. 5. (a) Thermal annealing of WBGs (written in 9% Yb-doped phosphate). At each temperature there are multiple data points taken over a 30 minute period. This is shown for temperatures of 145 °C and 265 °C in (b).